The present invention relates to a voltage controlled oscillator used in a phase locked loop circuit.
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FIG. 1 illustrates a prior art voltage controlled oscillator (VCO) 50. Inductors 1a and 1b each have a first end supplied with power Vcc and a second end connected to variable capacitors 2a and 2b. A control voltage VT is supplied between the variable capacitors 2a and 2b. The capacitances of the variable capacitors 2a and 2b vary in accordance with the control voltage VT.
A fixed capacitor 3 and the variable capacitors 2a and 2b are connected in parallel between the inductors 1a and 1b. The collector of an NPN transistor Tr1 is connected to a second end of the inductor 1a. The collector of an NPN transistor Tr2 is connected to a second end of the inductor 1b. 
The base of the transistor Tr1 is connected to the collector of the transistor Tr2. The base of the transistor Tr2 is connected to the collector of the transistor Tr1. The emitters of the transistors Tr1 and Tr2 are connected to a current source 4.
Complementary output signals OUT and XOUT are respectively output from the collectors of the transistors Tr1 and Tr2 as an output signal OUTVCO of the VCO 50.
The VCO 50 oscillates in accordance with the supply of power VCC. The frequency fVCO of the output signal OUTVCO of the VCO 50 is represented by the next equation.ƒVCO=1/2π√{square root over (LC)}
In the equation, L represents the inductance values of the inductors 1a and 1b, and C represents the sum of the capacitances of the variable capacitors 2a and 2b and the fixed capacitor 3. When the capacitances of the variable capacitors 2a and 2b vary in accordance with the control voltage VT, the frequency fVCO of the output signal OUTVCO is varied.
When incorporating the VCO 50, the oscillation frequency band of the VCO 50 must be widened to widen the logic range band of the PLL circuit. However, as shown in FIG. 2, when widening the oscillation frequency band of the VCO 50, deterioration in carrier-noise (CN) characteristic and deterioration in the linearity of the oscillation frequency fVCO relative to changes in the control voltage VT occurs.
As shown in FIG. 3, when narrowing the oscillation frequency band of the VCO 50, deterioration of the CN characteristic and linearity is prevented. However, this narrows the logic range of the PLL circuit. Further, when the oscillation frequency band is not included in a predetermined range due to manufacturing differences, the margin for manufacturing differences may become small.